A solid electrolytic capacitor as a rule is comprised of a porous metal electrode, an oxide layer located on the metal surface, an electrically conducting solid material that is incorporated in the porous structure, an outer electrode, such as for example a silver layer, as well as further electrical contacts and an encapsulation.
Examples of solid electrolytic capacitors are tantalum, aluminum, niobium and niobium oxide capacitors with charge transfer complexes, manganese dioxide electrolytes or polymer/solid electrolytes. The use of porous bodies has the advantage that, on account of the large surface, very high capacitance densities, i.e. high electrical capacitances in a small volume, can be achieved.
On account of their high electrical conductivity π-conjugated polymers are particularly suitable as solid electrolytes. π-conjugated polymers are also termed conducting polymers or synthetic metals. They are becoming increasingly important economically since polymers have advantages over metals as regards processability, weight and the specific adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important and technically used polythiophene being poly(3,4-ethylene-1,2-dioxy)thiophene, often also termed poly(3,4-ethylenedioxythiophene), since it exhibits a very high conductivity in its oxidized form.
Technical developments in electronics increasingly require solid electrolytic capacitors with a very low equivalent series resistance (ESR). The reasons for this are for example decreasingly logic circuit voltages, a higher integration density and rising clock frequencies in integrated circuits. In addition a low ESR also reduces the energy consumption, which is advantageous in particular for mobile, battery-operated uses. It is therefore desirable to reduce further as far as possible the ESR of solid electrolytic capacitors.
European patent application EP-A 340 512(=U.S. Pat. No. 4,910,645) describes the production of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymer, produced by oxidative polymerization in situ, as a solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene) as a replacement for manganese dioxide or charge transfer complexes in solid electrolytic capacitors reduces, on account of the higher electrical conductivity, the equivalent series resistance of the capacitor and improves the frequency response.
In Japanese patent specification JP-B 3296727 a special process is described for the production of solid electrolytic capacitors. For this, a capacitor anode is impregnated in a solution of oxidizing agent and 3,4-ethylene-1,2-dioxythiophene and the mixture is then polymerized at an atmospheric humidity of greater than 70% and at a temperature between 30° C. and 50° C. The disadvantage of this process is that the films produced thereby on the outer anode surface are not sufficiently stable with respect to external stresses. The capacitors produced by this process therefore exhibit a high leakage current and high ESR values.
Apart from a low ESR modern solid electrolytic capacitors require a low leakage current and a good stability with respect to external stresses. High mechanical stresses occur in particular during the production process in the encapsulation of the capacitor anodes, which may greatly increase the leakage current of the capacitor anode.
Stability with respect to such stresses and thus a lower leakage current can be achieved in particular by a ca. 5–50 μm-thick outer layer of conducting polymers on the capacitor anode. Such a layer serves as a mechanical buffer between the capacitor anode and the electrode on the cathode side. This prevents the electrode, for example under mechanical stress, coming into direct contact with the anode or damaging the latter and thereby increasing the leakage current of the capacitor. The conducting polymeric outer layer itself exhibits a so-called self-healing ability: minor defects in the dielectric on the outer anode surface that occur despite the effect of the buffer are thereby electrically insulated, with the result that the conductivity of the outer layer at the defect site is destroyed by the electric current.
The formation of such an outer layer by means of in situ polymerization is very difficult. In this connection the layer formation requires very many coating cycles. Due to the large number of coating cycles the outer layer is very non-homogeneous, and in particular the edges of the capacitor anode are often insufficiently covered.
If oxidizing agents and monomers are applied jointly in the form of mixtures to the capacitor anode in order to produce a polymeric outer layer, then the polymeric outer layer flakes off from the capacitor anode on account of inadequate adhesion even before a sufficiently thick and homogeneous outer layer could be formed. This leads to high ESR values and high leakage currents.
Japanese patent application JP-A 2003-188052 discloses that a homogeneous edge coverage is possible by means of a sequential application of oxidizing agents and monomers as well as a complicated matching of the process parameters. This however makes the production process very expensive and complicated and susceptible to interruptions.
A compact outer layer with good edge coverage can be achieved by electrochemical polymerization. Electrochemical polymerization requires however that a conducting film is first of all deposited on the insulating oxide layer of the capacitor anode and this layer is then electrically contacted for each individual capacitor. This contacting procedure is very complicated under mass production conditions and can damage the oxide layer.
A capacitor with a compact outer layer can also be produced if the capacitor anode is first of all coated, as described for example in the patent specifications EP-A 340 512 (=U.S. Pat. No. 4,910,645) or JP-B 3296727 mentioned above, by means of in situ polymerization, following which an outer layer is produced by the application of a formulation comprising conducting polymers with binder materials. Since however conducting polymer is already deposited on the outer anode surface in the in situ polymerization, the outer layer produced from the formulation is not in direct contact with the anode surface but adheres to the in situ layer. The poor adhesion of the in situ layer to the anode surface then leads in turn to localized peeling of the outer film and thus to a higher ESR. The sole application of a well-adhering formulation of conducting polymers with binder materials without prior in situ coating is not sufficient since the conductivity of the formulation is too low to achieve a low ESR or the formulation does not penetrate the pore structure at all.
There is thus a need for a simple process for the production of solid electrolytic capacitors that are characterized by a low equivalent series resistance (ESR), a low leakage current and a compact polymer outer layer with good edge coverage.
The object was therefore to discover suitable processes by means of which such solid electrolytic capacitors can be produced.